Wrought aluminium-magnesium alloy product

ABSTRACT

The invention relates to a wrought aluminium-magnesium alloy product, having a composition (in wt. %) of: 3.1&lt;Mg&lt;4.5; 0.4&lt;Mn&lt;0.85; 0.4&lt;Zn&lt;0.8; 0.06&lt;Cu&lt;0.35; Cr&lt;0.25; Fe&lt;0.35; Si&lt;0.2; Zr&lt;0.25; Ti&lt;0.3; others≦0.05, each up to a total of max. 0.15, and balance aluminium.

[0001] The invention relates to a wrought aluminium alloy product, inparticular a rolled aluminium-magnesium alloy product. In anotheraspect, the invention relates to a welded structure, comprising such awrought aluminium alloy product.

[0002] Such aluminium-magnesium products are known to be used in theform of sheets or plates in the construction of welded or joinedstructures such as marine, automotive, and aircraft applications,storage tanks, pressure vessels, land or marine structures. Wroughtproducts are products that have been subjected to mechanical working bysuch processes as rolling, extruding, or forging. Rolled products mayhave a gauge to about 200 mm.

[0003] A well-known standard alloy having appropriate formability andweldability, is the Aluminium Association (AA)5454 alloy having thenominal composition, in wt. %:

[0004] 2.4≦Mg≦3.0

[0005] 0.50≦Mn≦1.0

[0006] Zn≦0.25

[0007] Cu≦0.10

[0008] 0.05≦Cr≦0.20

[0009] Fe≦0.40

[0010] Si≦0.25

[0011] Ti≦0.20

[0012] others≦0.05 each, and in total max. 0.15;

[0013] and balance aluminium.

[0014] Although the formability and weldability of the standard AA5454alloy are sufficient for many applications, the alloy does not meet thedesired higher strength levels. There is a constant drive towarddown-gauging, for which a basic requirement is to increase the strength.With the fairly low Mg-level, the alloy product is not susceptible tointergranular corrosion (“IGC”).

[0015] Standard aluminium alloy AA5083, which has a Mg content ofbetween 4.0 and 4.9 wt. %, is known to be susceptible to IGC. This ishighly undesirable, because an alloy product that has low resistanceagainst IGC cannot be used in a reliable manner, in particular atservice temperatures above 65° C.

[0016] It is an object of the invention to provide an aluminium alloywrought product having a higher strength, when compared in a similartemper, than the standard AA5454 alloy, and a formability that is atleast as good as the formability of the standard AA5454 in a similartemper, and that is also at least as well weldable.

[0017] It is another object of the invention to provide a wrought alloyproduct that has similar mechanical properties, when compared in asimilar temper condition, as the standard AA5083 alloy, in particularregarding strength and formability, and at the same time has an improvedresistance against IGC over the standard AA5083, at lest when the AA5083is in an O-temper condition.

[0018] It is yet another object of the invention to provide a wroughtalloy product that has similar mechanical properties, when compared in asimilar temper condition, as the standard AA5083 alloy, in particularregarding strength and formability, and at the same time has aresistance against IGC that is similar to that of a standard AA5454alloy, or better.

[0019] According to the invention there is provided analuminium-magnesium alloy in the form of a wrought product, having acomposition (in wt. %) of:

[0020] 3.1<Mg<4.5

[0021] 0.4<Mn<0.85

[0022] 0.4<Zn<0.8

[0023] 0.06<Cu<0.35

[0024] Cr<0.25

[0025] Fe<0.35

[0026] Si<0.2

[0027] Zr<0.25

[0028] Ti<0.3

[0029] impurities each≦0.05, and a total of max. 0.15;

[0030] and balance aluminium.

[0031] By the invention can be provided a wrought alloy product that hasa higher strength than AA5454. Also, the welded joints can have a higherstrength than the standard AA5454 welds when using the same or similartemper material. Surprisingly, the product according to the inventionhas good resistance against corrosion, in particular against IGC. It hasbeen thought in the past that resistance against IGC is normally reducedwhen the Mg content exceeds about 3.0 wt. %, but the resistance againstIGC of the product according to the invention is high compared to knownAA5xxx-series wrought products with a Mg content exceeding 3.1 wt. %.

[0032] It has been found that the wrought alloy product according to theinvention has a weight loss of less than 15 mg/cm², in particular evenless than 10 mg/cm², at in the best examples even less than 6 mg/cm²,when tested in accordance with ASTM G67, after sensitising at atemperature of 100° C. during 100 hours.

[0033] It is believed that the improved balance of properties availablewith the invention, particularly the higher strength in combination withthe improved corrosion resistance, in particular against IGC, resultsfrom the balanced combination of the alloying elements Mg, Mn, Zn, andCu in the given ranges. Particularly, it is believed that the Cu and theZn levels in the ranges according to the invention contribute to thestrength of the wrought aluminium alloy product, and at the same timeoptimise the resistance against corrosion, in particular the resistanceagainst IGC and exfoliation corrosion.

[0034] Mg is the primary strengthening element in the alloy product. Mglevels above 3.1 wt. % do provide the required strength. The amount ofMg should not exceed 4.5 wt. %, in order to make possible to obtain therequired corrosion resistance. Preferably, the amount of Mg does notexceed 4.4 wt. % which is good for the fracture toughness and for theease of manufacturing. In an embodiment, the amount of Mg is more than3.6 wt. %. Herewith the desired balance of characteristics of the alloyproduct is better achieved. It is particularly surprising that evenhaving more than 3.6 wt. % Mg, but less than 4.5 wt. %, the resistanceagainst IGC remains sufficiently good. Preferably, the lower limit of Mgis 3.8 wt. %, and more preferably the amount of Mg is higher than 4.05wt. %, by which an alloy product is provided with a better optimisedbalance of tensile strength and yield strength, and corrosion behaviour.

[0035] Mn is an essential alloying element. In combination with Mg, Mnprovides the strength in the wrought product as well as in the weldedjoints of the alloy. Mn levels below 0.4 wt. % cannot provide sufficientstrength to the welded joints of the alloy. Above 0.85 wt. %, thedesirable formability characteristics of the alloy product suffer toomuch. In an embodiment, the amount of Mn does not exceed 0.69 wt. %, andpreferably does not exceed 0.6 wt. %, to further optimise theformability in the balance of mechanical properties. It is moreoverpreferred that the amount of Mn is more than 0.45 wt. %, more preferablyMn is more than 0.5 wt. %, in order to obtain sufficient strength in thebalance of mechanical properties.

[0036] Zn is an important additive for corrosion resistance of the alloyand for the formability. At least 0.4 wt. % Zn addition is required inorder to achieve sufficient resistance against IGC. At a Zn contentabove 0.8 wt. %, the uniform elongation is significantly reduced andthereby adversely affecting the formability of the alloy product, e.g.the reverse bendability is adversely affected. In an embodiment, theamount of Zn does not exceed 0.75 wt. %, for good insurance againstexfoliation in combination with a good formability. It is preferred thatthe amount of Zn does not exceed 0.6 wt. %, in order to further optimisethe balance of desired characteristics of the alloy product.

[0037] Surprisingly, in a narrow range Cu has been found to increase theresistance against IGC even though the Mg content is relatively high.Normally, a deliberate Cu addition is avoided in aluminium-magnesiumalloys of this type, since it is thought to harm the resistance againstcorrosion. When Cu is present above 0.06 wt. %, a positive effect hasbeen found on the resistance against corrosion. However, Cu should bekept below 0.35 wt. % in order to avoid an adverse effect on theresistance against corrosion, in particular in the resistance againstpitting corrosion. In an embodiment, the amount of Cu is more than 0.075wt. %, and more preferably more than 0.1 wt. %. Herewith a goodresistance against IGC is better save guarded. In embodiment, the amountof Cu does not exceed 0.24 wt. %. Herewith the balance of desiredcharacteristics is better achieved. Preferably the amount of Cu notexceeding 0.18 wt. %, in order to preserve the corrosion resistance in aweld zone. It is more preferred if Cu does not exceed 0.15 wt. %, tobetter ensure good corrosion resistance in a weld zone. Also, thegeneral resistance against IGC in the product is improved.

[0038] Fe is not an essential alloying element, but it is usuallypresent in the alloy because it is often present in source material. Feforms Al—Fe—Mn compounds during casting, thereby limiting the beneficialeffects of Mn. Therefore Fe must not be present in an amount of 0.35 wt.% or more. For the mechanical properties of the product, in particularthe formability, the amount of Fe is preferably kept below 0.2 wt. %.

[0039] Si is not an essential alloying element. It also combines with Feto form coarse Al—Fe—Si phase particles which can affect the fatiguelife and fracture toughness of the welded joints of the alloy. For thisreason, the Si level is kept to a maximum of 0.2 wt. %. Preferably theamount of Si is kept to a maximum of 0.12 wt. %, and more preferably ata maximum of 0.10 wt. % in order to better ensure a favourableformability characteristics of the alloy product.

[0040] Zr is not essential for achieving the improved corrosionperformance in the alloy product, but it can have an effect to achieve afine grain refined structure in the fusion zone of welded joints. Zrlevels above 0.25 wt. % are to be avoided, since that tends to result invery coarse needle-shaped primary particles with decrease in case offabrication of the alloys and in the formability of the wrought alloyproduct.

[0041] Zr may cause to form undesirable coarse primaries, in particulartogether with Ti. In an embodiment of the invention, the amount of Zrdoes therefore not exceed 0.05 wt. %. Moreover, it may be favourable tokeep Zr out of scrap material for specific recycling reasons. To thisextend, it is more preferred to limit the presence of Zr to less than0.01 wt. %.

[0042] Ti is often used as a grain refiner during solidification of bothcast ingots and welded joints produced using the alloy product of theinvention. This effect is obtained with a Ti content of less than 0.3wt. %, and preferably less than 0.15 wt. %. Ti may be replaced in partor in hole by V in the same compositional range.

[0043] Cr is an optional alloying element, that improves further thecorrosion resistance and strength of the alloy product. However, Crlimits the solubility of Mn and, if present, also that of Zr. Therefore,to avoid formation of undesirable coarse primaries, the Cr level mustnot be more than 0.25 wt. %. In a preferred embodiment, the Cr ispresent in a range of 0.06 to 0.2 wt. %, and more preferably in therange of 0.11 to 0.2 wt. %.

[0044] It is convenient for many applications if the wrought aluminiumalloy product is a rolled aluminium alloy product, such as a sheet or aplate product.

[0045] The aluminium alloy product may be provided in a wide range ofgauges, for example up to 200 mm, but a preferred gauge for thealuminium alloy product according to the invention is in the range of0.7 to 4 mm, and more preferably in the range of 1.6 to 2.4 mm.

[0046] The alloy product according to the invention can be delivered invarious temper conditions. However, for the group of applications forwhich the alloy product is ideally suited, it should be a temper similarto a soft worked temper, also known as an “O”-temper, or, in case ofsheets or plates, a light “H”-temper such as for example H111-temper.

[0047] In an embodiment of the invention, the tensile strength of thealloy product in O-temper at 50° C., even after holding the product atthis temperature for 1,000 hours, is at least 280 MPa, and the proofstrength at 50° C., even after holding the product at this temperaturefor 1000 hours, is at least 130 MPa. Herewith is achieved that thewrought alloy product has sufficiently good and sustainable mechanicalproperties at operating conditions.

[0048] Preferably, the proof strength of the wrought alloy product at100° C., even after holding the product at this temperature for 1,000hours, is at least 120 MPa, and more preferably at least 125 MPa.Herewith is achieved that the wrought alloy product has sufficientlygood and sustainable mechanical properties at more severe operatingconditions.

[0049] The elongation value A50 of the product according to theinvention is at least 20%. Herewith is achieved that the elongation,which is a measure for the formability, is at least as good as that of astandard AA5083 alloy in a similar, soft worked, temper condition.

[0050] The invention further relates to a welded structure comprising atleast one section of the product according to one of the above describedembodiments. The alloy according to one or more embodiments of theinvention is eminently suitable for application in such a weldedstructure due to its excellent weldability, and its high strength in aweld zone.

[0051] The invention further relates to a pressure vessel, in particulara welded pressure vessel, comprising a shell that comprises the wrought,in particular rolled, aluminium-magnesium alloy product according to anembodiment as is described above. Due to the increased strength, suchpressure vessel can be down-gauged to have a lower weight. Moreover, thecorrosion properties can be improved. The pressure vessel according tothis aspect of the invention can be used at a higher servicetemperature, in particular above 65° C.

[0052] The invention further relates to an air suspension system, inparticular for an automotive vehicle, comprising the above describedpressure vessel.

[0053] The invention further relates to a braking system for a transportvehicle, comprising the above described pressure vessel.

[0054] The invention further relates to a transport vessel comprisingsuch a braking system or air suspension system. The transport vehiclecan in particular be an automotive vehicle comprising the abovementioned braking system. Because of the good corrosion properties, thebraking system or the air suspension system can be mounted in an exposedway to the vehicle.

[0055] In another aspect, the invention relates to a method of producinga wrought aluminium alloy product comprising the subsequent steps of:

[0056] i. providing an intermediate alloy product having a compositionaccording to the inventive composition mentioned above;

[0057] ii. cold working the intermediate alloy product to a final gaugeto obtain an intermediate wrought product;

[0058] iii. annealing the intermediate wrought product by heating theproduct at a heating rate in the range of 2 to 200° C./sec, holding theproduct at a soaking temperature in the range of 480 to 570° C. for aduration of up to 100 sec, followed by a cooling at a cooling rate inthe range of 10 to 500° C./sec to below a temperature of 150° C.

[0059] By this method it is achieved that the positive influence of Cuon the resistance against IGC is fully exploited. Although the wroughtaluminium alloy product has good properties when other annealing schemesare applied, it is believed that the positive influence of Cu on thecorrosion properties is enhanced by the annealing scheme of step iii.

[0060] This annealing scheme can be applied in a continuous annealingfacility. The required heating rates can be achieved, for example, byhomogeneous heating by means of inductive heating. This gives improvedmechanical properties to the sheet. The required heating and coolingrates may be possible to achieve for gauges up to 4 mm, but the requiredrates are particularly feasible when the gauge is thinner than 2.4 mm.

[0061] Particularly favourable results have been obtained in anembodiment of the method wherein the soaking temperature is in the rangeof between 520 and 550° C.

[0062] The balance of characteristics of the product produced by themethod is found to be better optimised in an embodiment wherein theproduct is held at the soaking temperature for a duration of up to 40sec.

[0063] In an embodiment, the heating rate is at least 50° C./sec, andpreferably at least 80° C./sec. Herewith, the balance between themechanical properties and the resistance against IGC has been found tobe more favourable. This is especially the case when the cooling rateafter soaking is at least 100° C./sec.

[0064] The invention will now be explained with reference to laboratoryexperiments.

[0065] Various slabs were cast having chemical compositions as shown inthe following Table 1, balance aluminium. The slabs A and B correspondto a standard AA5454 alloy and a standard AA5083 alloy, respectively,and Slabs C to F are according to the invention. TABLE 1 compositions(in wt %) of the cast slabs (balance Al and impurities) Slab Inv. Mg MnZn Cu Cr Fe Si Zr Ti A No 2.7 0.75 0.02 0.005 0.10 0.30 0.16 0.001 0.02B No 4.5 0.50 0.03 0.005 0.10 0.31 0.16 0.001 0.015 C Yes 4.29 0.50 0.540.085 0.14 0.14 0.04 0.001 0.02 D Yes 4.29 0.50 0.54 0.085 0.14 0.140.04 0.001 0.02 E Yes 4.31 0.52 0.51 0.18 0.15 0.19 0.10 <0.01 0.02 FYes 4.31 0.52 0.51 0.18 0.15 0.19 0.10 <0.01 0.02

[0066] The processing of the slabs A to C and E comprised a pre-heat, ahomogenisation anneal during 5 hours at a temperature of 540° C., hotrolling whereby the exit temperature was about 330° C., followed by coldrolling with 60% cold reduction and finally soft annealing in batchanneal at a temperature of 330° C. during 1 hour. The processing of slabD and F was identical to those of A to C and E, with the exception ofthe final soft anneal, which was a continuous anneal for 10 sec. at 530°C. Final gauges were 3 mm, and the plates were delivered in H111-temper.

[0067] These products were tensile tested according to EN 10002, and theresults for the parallel (∥) and perpendicular (⊥) directions are givenin Table 2, and whereby (-) stands for not tested. TABLE 2 Tensilestrength (“UTS”), 0.2% Proof strength (“PS”), Elongation (“A50”)Direction of UTS PS A50 Alloy testing [MPa] [MPa] [%] A ∥ 237 108 19(AA5454) ⊥ 233 107 20 B ∥ 299 149 19 (AA5083) ⊥ 293 147 21 C ∥ 299 14119 ⊥ 293 139 21 D ∥ 303 146 22 ⊥ 294 145 23 E ∥ 300 141 19 ⊥ 296 141 21F ∥ 303 148 21 ⊥ 297 146 21

[0068] The alloy products have been subjected to a weight loss testaccording to ASTM G67 after sensitising at 100° C. for a duration of 100hours in H111-temper condition.

[0069] Results are shown in Table 3. TABLE 3 Weight loss (in mg/cm²)after sensitising. A B C D E F 100 hr at 100° C. 6 36 22 14 11 7

[0070] This indicates that the corrosion resistance of products C to Fare much better than of the standard AA5083 alloy (B). Products D to Fare below 15 mg/cm², which is according to ASTM-G67 the upper limit fora product quality not susceptible to IGC. At the same time, as can beseen from Table 2, the elongation (A50) of product C to F aresufficiently high.

[0071] The resistance against IGC of products D and F in the presentlyused sensitising conditions show an improvement over that of C and Erespectively, apparently by using the continuous anneal the corrosionresistance of the product is improved. It is expected that under moresevere sensitising conditions the difference is more clearly visible.

[0072] Alloy product C was also tested in non sensitised condition, andin that condition it lost only 3 mg/cm².

[0073] Alloy products C to F were welded without any problem using TIGwelding under standard conditions.

[0074] In an additional test series, the influence of Cu on thecorrosion resistance was tested. Some additional slabs were cast havingthe chemical compositions as shown in the following table 4, balancealuminium.

[0075] The processing of the additional alloys was identical to theprocessing of alloys D and F, i.e. with a final soft anneal as acontinuous anneal. TABLE 4 composition (in wt %) of the cast slabs (bal.Al and impurities) Slab Inv. Mg Mn Zn Cu Cr Fe Si Zr Ti G No 4.18 0.500.50 0.02 0.15 0.17 0.10 <0.01 0.02 H Yes 4.24 0.50 0.50 0.12 0.16 0.200.10 <0.01 0.02 J Yes 4.29 0.50 0.51 0.30 0.15 0.19 0.10 <0.01 0.02 K No4.30 0.50 0.51 0.42 0.15 0.20 0.10 <0.01 0.02

[0076] The alloy products have been subjected to a weight loss testaccording to ASTM G67 after sensitising at 100° C. for a duration of 100hours in H111 temper condition. The alloy products have also beensubjected to an ASSET test according to ASTM G66 after welding, followedby sensitising at 100° C. for a duration of 100 hours. The weld was aTIG weld using AA5183 as filler wire. Results are shown in table 5. TheASSET results correspond to the Heat Affected Zone (HAZ), because herethe most severe attack is usually found. TABLE 5 Weight Loss (in mg/cm²)and ASSET result after sensitising. Alloy % Cu WL [mg/cm²] ASSET resultin HAZ G 0.02 30  N D  0.085 14  PA H 0.12 8 PA F 0.18 7 PB J 0.30 6 PBK 0.42 6 PC

[0077] According to ASTM G67 the upper limit for a product quality notsusceptible to IGC is 15 mg/cm². In ASTM G66 the range to classify theresults is given, but limits for acceptable or not acceptable are notspecified. However, for a person skilled in the art, it is clear thatpitting A is still acceptable whereas pitting C in unacceptable. PittingB is for most applications still acceptable.

[0078] The results indicate that the resistance against IGC increaseswith increasing Cu content, but at the same time the resistance againstpitting decreases. For a Cu level of 0.30 wt. % and lower, theresistance against pitting is acceptable and better than acceptable. Theweight loss is thought to measure below 15 mg/cm² when the Cu level isabove about 0.075 wt. %.

[0079] Based on these results it is concluded that the broadestoperational window is found with Cu levels between 0.06 and 0.35 wt. %.Preferably the amount of Cu does not exceed 0.18 wt. % in order topreserve the corrosion resistance in a weld zone.

In the claims
 1. Wrought aluminium-magnesium alloy rolled product,having a composition (in wt. %) of: 3.1<Mg<4.5 0.4<Mn<0.85 0.4<Zn<0.80.06<Cu<0.35 Cr<0.25 Fe<0.35 Si<0.2 Zr<0.25 Ti<0.3 impurities each≦0.05,total of max. 0.15, and balance aluminium.
 2. Product according to claim1, wherein the amount of Mg is more than 3.6 wt. %.
 3. Product accordingto claim 1, wherein the amount of Mg is at least 3.8 wt. %.
 4. Productaccording to claim 1, wherein the amount of Mg is at least 4.05 wt. %.5. Product according to claim 1, wherein the amount of Mg does notexceed 4.4 wt. %.
 6. Product according to claim 1, wherein the amount ofMn does not exceed 0.69 wt. %.
 7. Product according to claim 1, whereinthe amount of Mn does not exceed 0.6 wt. %.
 8. Product according toclaim 1, wherein the amount of Mn is more than 0.45 wt. %.
 9. Productaccording to claim 1, wherein the amount of Mn is more than 0.5 wt. %.10. Product according to claim 1, wherein the amount of Zn does notexceed 0.75 wt. %.
 11. Product according to claim 1, wherein the amountof Zn does not exceed 0.6 wt. %.
 12. Product according to claim 1,wherein the amount of Fe is less than 0.20 wt. %.
 13. Product accordingto claim 1, wherein the amount of Cu is more than 0.075 wt. %. 14.Product according to claim 1, wherein the amount of Cu is more than 0.1wt. %.
 15. Product according to claim 1, wherein the amount of Cu isless than 0.24 wt. %.
 16. Product according to claim 1, wherein theamount of Cu is less than 0.18 wt. %.
 17. Product according to claim 1,wherein the amount of Cu is less than 0.15 wt. %.
 18. Product accordingto claim 1, wherein the amount of Si is maximum 0.12 wt. %.
 19. Productaccording to claim 1, wherein the amount of Si is maximum 0.10 wt. %.20. Product according to claim 1, wherein the amount of Zr does notexceed 0.05 wt. %.
 21. Product according to claim 1, wherein the amountof Zr is less than 0.01 wt. %.
 22. Product according to claim 1, whereinthe amount of Cr is in range of 0.06 to 0.2 wt. %.
 23. Product accordingto claim 1, wherein the amount of Cr is in a range of 0.11 to 0.2 wt. %.24. Product according to claim 1, wherein the product is provided in anO-temper condition.
 25. Product according to claim 1, wherein the weightloss after sensitizing for 100 hours at 100° C. is less than 15 mg/cm²when tested against intergranular corrosion in accordance with ASTM G67.26. Product according to claim 1, wherein the weight loss aftersensitizing for 100 hours at 100° C. is less than 10 mg/cm² when testedagainst intergranular corrosion in accordance with ASTM G67.
 27. Productaccording to claim 1, wherein the product has an elongation A50 of atleast 20%.
 28. Product according to claim 1, wherein the product has agauge in the range of 0.7 to 4 mm.
 29. Product according to claim 1,wherein the product has a gauge in the range of 1.6 to 2.4 mm. 30.Product according to claim 1, having a composition (in wt. %) consistingof: 3.1<Mg<4.5 0.4<Mn<0.85 0.4<Zn<0.8 0.06<Cu<0.35 Cr<0.25 Fe<0.35Si<0.2 Zr<0.25 Ti<0.3 impurities each≦0.05, total of max. 0.15, andbalance aluminium.
 31. Welded structure, comprising at least one sectionof the product according to claim
 1. 32. Pressure vessel comprising ashell that comprises the wrought aluminium-magnesium alloy productaccording to claim
 1. 33. Method of producing a wrought aluminium alloyproduct comprising the steps: (i) providing an intermediate alloyproduct having a composition according to the composition according toclaim 1; (ii) cold working the intermediate alloy product to a finalgauge to obtain an intermediate wrought product; (iii) annealing theintermediate wrought product by heating the product at a heating rate inthe range of 2 to 200° C./sec, holding the product at a soakingtemperature in the range of 480 to 570° C. for a duration of at most 100sec, followed by a cooling at a cooling rate in the range of 10 to 500°C./sec to below a temperature of 150° C.
 34. Method according to claim33, wherein the product is held at the soaking temperature in the rangeof 480 to 570° C. for a duration of at most 40 sec.
 35. Method accordingto claim 33, wherein the soaking temperature during step (iii) is in therange of 520 to 550° C.
 36. Method according to claim 33, wherein theheating rate during step (iii) is at least 50° C./sec.
 37. Methodaccording to claim 33, wherein the heating rate during step (iii) is atleast 80° C./sec.
 38. Method according to claim 33, wherein the finalgauge of the aluminium sheet is at most 4 mm.
 39. Method according toclaim 33, wherein the final gauge of the aluminium sheet is at most 2.4mm.
 40. Method according to claim 33, wherein the final gauge of thealuminium sheet is in a range of 1.6 to 2.4 mm.